Decorative hard golden ceramic article

ABSTRACT

Disclosed is a decorative hard golden article comprising at least one member selected from titanium nitride and zirconium nitride as a main component and 0.1 to 30% by weight of at least one member selected from alumina (Al 2  O 3 ) and zirconia (ZrO 2 ). 
     In this decorative hard golden article, the corrosion resistance is highly improved while the flexural strength and hardness are maintained at high levels.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an improvement of the corrosionresistance in the decorative hard golden ceramic article comprisingtitanium nitride (TiN) or zirconium nitride (ZrN).

(2) Description of the Prior Art

Sintered alloys comprising a carbide, nitride or carbonitride of atransition metal of the Group IV or V, such as TiN, ZrN or TaC, as themain component and a binder metal such as Fe, Co, Ni or Mo has not onlya golden hue but also such excellent properties as high hardness andhigh strength. Accordingly, these sintered alloys are broadly used asdecorative articles in order to obtain sintered alloy having highhardness and high strength.

Sintering of TiN, ZrN or TaC alone is very difficult, and therefore, abinder metal as mentioned above is added as a sintering aid. Since thesintering aid is a metal element, corrosion of the metal componentpresent in the sintered alloy is readily advanced, and discoloration iscaused, for example, by sweat and the hue of the decorative article isimpaired.

SUMMARY OF THE INVENTION

We made research with a view to eliminating the above defect, and as theresult, it was found that if a powder of alumina (Al₂ O₃) and/orzirconia (ZrO₂) is added as a sintering aid at a specific ratio to astarting powder of TiN and/or ZrN and the resulting powder mixture ismolded and sintered, the corrosion resistance is improved and anexcellent decorative hard golden ceramic article is obtained. We havenow completed the present invention based on this finding.

It is therefore a primary object of the present invention to provide adecorative hard golden ceramic article excellent in the corrosionresistance.

More specifically, in accordance with the present invention, there isprovided a decorative hard golden ceramic article comprising at leastone member selected from titanium nitride and zirconium nitride as amain component and 0.1 to 30% by weight of at least one member selectedfrom alumina (Al₂ O₃) and zirconia (ZrO₂).

DETAILED DESCRIPTION OF THE INVENTION

In the decorative hard golden ceramic article of the present invention,alumina and/or zirconia is added as a sintering aid in an amount of 0.1to 30% by weight. If the amount incorporated of the sintering aid issmaller than 0.1% by weight or exceeds 30% by weight, sintering becomesdifficult. Accordingly, in the present invention, alumina and/orzirconia is incorporated in an amount of 0.1 to 30% by weight,preferably 2 to 20% by weight. Ordinarily, a smaller amount incorporatedof the sintering aid gives a golden color having a yellowish tint.

The particle size of the starting powder of alumina and/or zirconia issmaller than 2.0 microns, preferably smaller than 1.0 micron. Thestarting powder of alumina and/or zirconia may be added at the step ofpreparing the starting material powder. In the case where the amountincorporated of the sintering aid is relatively small, for example, 5%by weight or less, there may be adopted a method in which an aluminaball or zirconia ball is added to the starting material powder and apredetermined amount of the powder of alumina or zirconia isincorporated into the starting material powdery by abrasion.

According to the process for preparing the golden ceramic article of thepresent invention, a mixture formed by adding a powder of alumina and/orzirconia to a starting material powder of TiN and/or ZrN iscompression-molded and the molded mixture is sintered in a non-oxidizingatmosphere such as nitrogen or argon or in vacuum. The sinteringtemperature is preferably about 1600° to about 1900° C. In the casewhere sintering is carried out in a vacuum furnace, the degree of vacuumis adjusted to 10⁻² to 10⁻⁵ Torr. Thus, a decorative hard golden ceramicarticle excellent in the corrosion resistance can be obtained.

The present invention will now be described in detail with reference tothe following Example that by no means limits the scope of theinvention.

EXAMPLE

A starting powder comprising at least one member selected from aluminaand zirconia was added to a starting material powder comprising at leastone member selected from TiN and ZrN at a ratio shown in Table 1, andthe mixture was pulverized for about 60 hours in acetone or methanol.The pulverized mixture was dried and 4% by weight of paraffin as addedthereto, and the mixture was compression-molded under a pressure of 1.0ton/cm². After removal of the binder, sintering was carried out at 1750°to 1850° C. under a pressure of 10⁻⁴ Torr in a vacuum furnace. Thesintered product was subjected to mirror polishing, and the hue,flexural strength, Vickers hardness (Hv) and corrosion resistance of theproduct were tested. The flexural strength was measured according to thethree-point bending test method of JIS R-1601, and the corrosionresistance was determined according to the sweat resistance test methodin which artificial sweat having a composition equivalent to thecomposition of human sweat was used and the sample was dipped in thisartificial sweat. The obtained results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Sample Mixing Ratios (% by weight) of Components                              No.    TiN     ZrN     Al.sub.2 O.sub.3                                                                     ZrO.sub.2                                                                            Ni    Mo                                 ______________________________________                                         1*     99.94                 0.06                                             2*             99.95  0.05                                                    3     99.8                   0.2                                              4     99.5            0.5                                                     5*    99.5                           0.5                                      6     79.5    19.9           0.6                                              7     94.6                   5.4                                              8             94.2           5.8                                              9     20.1    68.7    6.2    5.0                                             10*    21.1    67.4                        11.5                               11             85.6    14.4                                                   12     80.6                   19.4                                            13             78.4    21.6                                                   14     25.2    51.3    16.2   7.3                                             15*            76.8                  10.5  12.7                               16             73.4           26.6                                            17     72.0            18.0   10.0                                            18     71.0            29.0                                                   19*    68.8            17.3   13.9                                            20*            67.9    11.7   20.4                                            ______________________________________                                        Properties                                                                    Sam- Flexural  Hardness  Corro-                                               ple  Strength  Hv        sion Re-                                             No.  (Kg/mm.sup.2)                                                                           (Kg/mm.sup.2)                                                                           sistance                                                                             Hue                                           ______________________________________                                         1*  20         800      ⊚                                                                     (insufficient sintering)                       2*  25         900      ⊚                                                                     (insufficient sintering)                       3   35        1300      ⊚                                                                     golden                                         4   30        1200      ⊚                                                                     golden                                         5*  70        1500      Δ                                                                              golden                                         6   30        1250      ⊚                                                                     whitish golden                                 7   65        1500      ⊚                                                                     golden                                         8   40        1400      ⊚                                                                     whitish golden                                 9   45        1330      ⊚                                                                     whitish golden                                10*  76        1500      X      whitish golden                                11   38        1270      ⊚                                                                     whitish golden                                12   55        1550      ⊚                                                                     dark golden                                   13   40        1300      ⊚                                                                     whitish golden                                14   42        1350      ⊚                                                                     whitish golden                                15*  80        1600      X      reddish golden                                16   45        1400      ⊚                                                                     whitish golden                                17   42        1350      ⊚                                                                     golden                                        18   36        1280      ⊚                                                                     golden                                        19*  20         750      ⊚                                                                     (insufficient sintering)                      20*  23         850      ⊚                                                                     (insufficient sintering)                      ______________________________________                                    

Each asterisked sample was a comparative sample outside the scope of thepresent invention.

The corrosion resistance was evaluated according to the following scale:

⊚: neither discoloration nor corrosion

Δ: discoloration

X: corrosion in addition to discoloration

As is seen from the results shown in Table 1, in each of samples 1, 2,19 and 20, sintering was insufficient, and therefore, after the mirrorpolishing, the polished surface was prominently porous and the gloss waspoor. In each of samples 5, 10 and 15 in which Ni or Mo was used as acomparative sintering aid, it was found at the corrosion test that thecorrosion was advanced, and foggy discoloration was observed.

From the foregoing experimental results, it will readily be understoodthat when alumina or zirconia is used as the sintering aid according tothe present invention, neither corrosion nor discoloration is caused atthe corrosion test and also the flexural strength and hardness are good.

When the samples of the present invention were subjected to the saltspray test (JIS Z-2371-76) using an aqueous solution of sodium chloride(4 W/V %) and the dipping tests using an acetic acid-nitric acidsolution, a concentrated sulfuric acid solution (50% by weight), ahydrofluoric acid solution (46% by weight), aqua regia and a potassiumferricyanide (10% by weight), it was found that each sample had a goodcorrosion resistance.

As is apparent from the foregoing description, the decorative hardgolden ceramic article of the present invention is improved in thecorrosion resistance over hard golden ceramic articles prepared by usingthe conventional metal sintering aids, while the flexural strength andhardness are maintained to practically satisfactory levels in the fieldof decorative articles, and that the decorative article of the presentinvention can be used valuably as a watch case, a medal, a bracelet, aring, a button, a pendant, a tapestry or other decorative member.

What is claimed is:
 1. A decorative hard golden ceramic articleconsisting essentially of at least one member selected from titaniumnitride and zirconium nitride as the main component, and 0.1 to 5% byweight of at least one member selected from alumina (Al₂ O₃) andzirconia (ZrO₂).
 2. A ceramic article as set forth in claim 1, whereinthe Vickers hardness (Hv) is at least 1200 Kg/mm².
 3. A ceramic articleas set forth in claim 1 wherein the flexural strength is at least 30Kg/mm².
 4. A method for making a decorative hard golden ceramic articlecomprising the steps of:(a) forming a powdered mixture consistingessentially of at least one member selected from titanium nitride andzirconium nitride as the main component, and 0.1 to 5% by weight of atleast one member selected from alumina (Al₂ O₃) and zirconia (ZrO₂); (b)molding said mixture into a desired shape; and (c) sintering the moldedmixture in a non-oxidizing atmosphere.
 5. The method according to claim4 wherein said mixture in step (a) is formed by grinding said maincomponent with a ball of alumina or zirconia.
 6. The method according toclaims 4 or 5 wherein, in step (c), said sintering is conducted in avacuum.
 7. A product produced according to the method of claim
 4. 8. Aproduct produced according to the process of claim 6.